To understand the sense in which charity is and is not a virtue, one needs a distinction between two senses of the word “virtue.” In a broad sense, any lasting disposition to good acts is a virtue; in this sense, charity is a virtue (see S.t., 2–2, q. 23, aa. 2–3). In another and more specific sense, a virtue is an aspect of character integrated around morally good choices, which constitutes an enduring disposition toward good acts. In this more specific sense, charity is not a virtue.
All love is a disposition toward fufillment (24‑A). However, not all love is a virtue; not all love is an enduring disposition, and not all love disposes toward humanly good fulfillment. For example, a passing love for a particular, sensible good is not a virtue, and even a lasting disposition, such as love of the feeling of alcoholic intoxication, will not be a virtue, since it is not in accord with human fulfillment. However, a love which both is lasting and disposes to actuations of one’s capacities which will truly be humanly fulfilling is a virtue in the broad sense.
Charity, although it primarily disposes one to fulfillment in divine life, also disposes the Christian to integral human fulfillment. Faith primarily is an act, but insofar as it is a lasting gift on God’s part and an enduring commitment on the part of the believer, it has the character of a lasting disposition; this disposition is good insofar as it is a disposition to accept charity and the life of friendship with God which flows from it. Thus faith is a virtue. Hope is an attitude of trustful confidence in God’s faithfulness; this attitude is necessary for life according to faith; hence, the constant disposition of hope in God also has the character of virtue.
Although faith, hope, and charity can be called “virtues” in the sense explained, it is important to realize that they dispose to more than human fulfillment. They affect choices and human moral actions, but they also affect other parts of the personality. While certain human acts, done with the help of grace, are necessary to prepare one to receive these virtues, faith, hope, and charity are acquired not by human acts, but by the gift of the Spirit (see DS 1525–31/797–800; see S.t., 1–2, q. 62, a. 1; 2–2, q. 6, a. 1; q. 24, a. 2).
Very often faith, hope, and charity have been thought of as if they were separate acts. It seems more nearly correct to think that in living faith, the three concur in a single act. St. Paul closely and dynamically links the three (see Rom 5.1–5; Gal 5.5–6). Also, there are suggestions in the First Epistle of John that faith is a consequence of love (see 1 Jn 5.1). Since faith, hope, and love are received together and since faith usually is considered the basis (see Rom 5.1–5), the priority of love can hardly be understood unless in living faith the three concur in one and the same act.
The act of faith-hope-love can be viewed in this way: By faith we accept the truth of God’s self-revelation; acceptance of his truth makes us hope with confidence that he will fulfill the personal relationship he has initiated; this hope enables us to be disposed in love to live according to his will and so come to perfect communion with him. The act also can be viewed in this way: By love we share in God’s own life and are disposed to fulfillment in it; the experience of this love and its dynamism makes us hope with confidence for fulfillment; this personal trust in God renders our faith in his revealed truth absolutely certain and unshakeable.
This second way of viewing the dynamic unity of the three aspects of our personal relationship with God could clarify how justifying faith differs from preparatory acts of faith, why believing in God out of love of him justifies, and also how God testifies and the Spirit convinces. For if the assent of living faith is the first human act we do out of the love of God poured forth in our hearts through the Holy Spirit, then I see no reason to suppose that anything other than the gift of this love is required to make us certain beyond human certitude that what appears to be divine revelation most surely and truly is so: God’s proposal of communion is surely true, for we find ourselves beginning to live in it.
Someone might ask: How can this view be reconciled with the Council of Trent’s teaching about the faith of a person in mortal sin? Trent definitively teaches that by every mortal sin one loses grace or charity, but not faith except by sin contrary to it (see DS 1544/808, 1577–78/837–38). The faith which remains when one is in mortal sin is “true faith, granted it is not a living faith” (DS 1578/838). One remains a child of God although a prodigal one. I think this teaching is a very important reason why Catholics have tended to suppose that there are three distinct acts of the theological virtues, not one single act.
My answer is that when one continues to believe in God while standing against him in sin, one’s act of faith obviously cannot be motivated by love. Yet on the account just now sketched, the past experience of God’s love still can have its effect by the medium of hope. The past experience of love together with God’s grace is sufficient to keep alive confidence in his mercy, and this confidence can preserve certitude that what is revealed truly is God’s absolutely trustworthy communication. Even if I sin mortally, I know who is unfaithful.